For this assignment, you will implement two short programs in AVR assembly. Both programs will require the use of memory access instructions, conditional branches and arithmetic with carry. You should start early to ensure that you can ask for help if you get stuck.
Although the AVR instruction set has a full slate of addition and subtraction instructions, there are very few multiplication instructions and no instructions for integer division. The objective of this assignment is to implement multiplication and division in AVR assembly to allow 16-bit numbers to be multiplied or divided (with both the quotient and remainder retained). Although there are several variants of the MUL instruction available, you will likely find it easier to write your solution using only addition and subtraction (since the MUL instructions have a peculiar format that does not map well onto this task).
Let A and B be 16-bit unsigned integers. The product AB requires at most 32 bits. The quotient A/B and remainder A%B each require at most 16 bits. For this assignment, your task is to write two AVR assembly programs:
• A program to compute the product AB and store it in memory as a 32-bit little-endian unsigned integer.
• A program to compute the quotient A/B and remainder A%B and store each value in memory as a 16-bit little-endian unsigned integer.
1 Code Templates
Since the only environment available for running AVR assembly (besides the AVR boards that we haven’t used yet) is the simulator, it is not really possible to write a complete program in the traditional sense, because there is no facility for input and output. Instead, two code templates have been posted on conneX: mul32.asm and divmod16.asm. In each template, the 16-bit input operands A and B are pre-loaded into two pairs of registers (R17:R16 for A and R19:R18 for B). Add your implementation of each algorithm to the indicated place in each template. You may change the loaded operand values for testing purposes, but do not move or change any other aspects of the ldi instructions. When your code has computed the result for each task, store the result in data memory (using the variables indicated in each code template). If you do not use the code templates provided to implement your solution, it will not be possible to mark your code and you will receive a mark of zero.
